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Gravitational Waves Could Help Scientists Spot Cosmic Events

On October 16, a group of scientists working with LIGO (the Laser Interferometer Gravitational-Wave Observatory) announced that they had managed to detect gravitational waves created by two colliding neutron stars, the densest stars known to exist. Before these stars collided, they began orbiting each other faster and faster — finally reaching roughly a third of the speed of light. This fast-paced, high-energy dance was powerful enough to actually “stir” space-time, according to Hunter Gabbard of the University of Glasgow. Gabbard suggested thinking about the process as acting like pancake batter in a blender.

This stirring caused ripples to spread out from the neutron stars, through the fabric of space at the speed of light. These ripples eventually passed through Earth, and that’s what the LIGO scientists were able to detect (I wrote about just how this detection process works in a previous blog).

An artist’s impression of gravitational waves generated by binary neutron stars. Credits: R. Hurt/Caltech-JPL via NASA.govThe gravitational wave passed through Earth for around 100 seconds, but it was so weak that it would have been impossible to detect without advanced scientific equipment. In fact, according to Jennifer Wright (also a Ph. D student at the University of Glasgow), even if you were near the event, you would likely “have been torn apart by the constant strong gravitational tidal forces produced by something as dense as a neutron star” before you were able to actually feel the wave passing through your body.

This observation also has a practical purpose for astronomers: it could help them pinpoint exactly when and where events like neutron star collisions will occur, letting them know where to point their telescopes. According to Simon Barke, a postdoctoral researcher at the University of Florida, “we can now predict WHEN and WHERE EM events will happen. We kind of knew a minute in advance when a Kilonova was to happen. and where. That’s absolutely amazing in my opinion. If we get the data flow faster from detection algorithm telescope pointing, we could catch it in time.”

Evan Goetz (Scientist, LIGO instrumentalist/astrophysics) waxed poetic when he described the experience: “Observing gravitational waves for the first time is like finally being able to hear the symphony of the Universe and observe the dynamics of the most energetic events.”

I joined a Reddit AMA to ask some scientists from LIGO to clarify what exactly this means, and how they were able to detect this phenomenon.

MCB: How long did this gravitational event last? Did the wave only pass by Earth for a moment, or did it take some time?

The gravitational wave was detectable by LIGO for approximately 100 seconds. The wave traveled through the Earth for this entire time, traveling at the speed of light.

[Noah Sennett, PhD, LIGO source modeling]

MCB: What is it exactly about the collision of the two neutron stars that caused the gravitational wave?

Great question! So space-time is exceptionally stiff. You can think of it as an incredibly taught trampoline mat. In order to create ripples (or gravitational waves) in space-time you need an incredibly dense object moving at high velocity. Neutron stars are one of the few objects that are dense enough to do this. When the two neutron stars get closer to one another they rotate around each other faster and faster until finally they merge flying around at about one third the speed of light! While they’re moving around they essentially ‘stir’ space-time, similar to a blender in a pancake mix (Why pancake mix? Because pancakes are awesome.), and radiate some of their energy away in the form of space-time ripples … otherwise known as gravitational waves!

— HG, PhD Student, LIGO Data Analysis, University of Glasgow

MCB: Do gravitational waves have any observable effect on time? How about on light passing through the wave?

Gravitational waves don’t affect the nature of light, however, they can affect the path that light takes. Einstein’s theory of general relativity describes Space as a curved surface, and light travels along specific paths on this surface. Since gravitational waves contract and stretch space itself, the path that a photon takes changes as the space around it contracts and stretches. ~ Rachel H, Fermi GBM grad student

MCB: Is it possible for two gravitational waves to interact with each other? Would they interact like any other wave? Would we be able to observe that?

Yes, they sure can. They interfere like any other wave. This is in fact responsible for a certain gravitational wave background in our data that we cannot resolve at low signal-to-noise ratio.

To observe them at high signal-to-noise ratio, we’d have to be lucky enough that two pulses/chirps arrive at our detectors at the same time!

[Avneet Singh, Doctoral Researcher, Albert-Einstein-Institut]

MCB: Is there a theoretical limit to how strong a gravitational wave can be? Or how weak it can be before there’s no wave at all?

General relativity (Einstein’s theory of gravity) which describes gravitational waves is a classical theory. Generally in classical theories there aren’t any hard upper or lower bounds (as opposed to quantum theories which do suggest bounds). So from general relativity point of view there are no upper or lower limits on the strength of gravitational waves. Larger the mass stronger the gravitational waves and smaller the mass weaker the gravitational waves. However if the gravitational waves are too weak we won’t be able to detect them with our current detectors.

[SK, Scientist, LIGO Livingston observatory]

MCB: On Earth, this particular gravitational wave event was only observable with instruments. Would there be any point, closer to the source of the wave, where a person could hypothetically observe the effects of the gravitational wave without instruments?

So this is a tricky one… I think that if you were close to a neutron star merger you would notice the change in the local gravitational field acting on you from the gravitational waves but probably before getting to this point close to the star you would have been torn apart by the constant strong gravitational tidal forces produced by something as dense as a neutron star. The wave is the ripple in this gravitational field.

Neutron stars have such high constant gravitational fields in the first place that light close to them gets highly distorted around them.

This isn’t even mentioning all the terrible effects from the radiation produced as the two stars merge.

[JW, PhD student, experimental interferometry]

MCB: How did LIGO come up with its process for detecting gravitational waves

The idea for detecting gravitational waves using laser interferometers grew out of thought experiments and conceptual designs in the 60s and 70s. Joseph Weber was the first and early pioneer of gravitational wave detectors, but his experiments used solid aluminum cylinders that would ring like a bell should a gravitational wave with only the correct frequency pass through them. Large scale laser interferometers, on the other hand, would have a better sensitivity and wider frequency sensitivity than these so-called “bar detectors”. Several groups independently thought about interferometers as gravitational wave detectors, because they are well-suited to measure the alternately stretching and compression of space-time as a gravitational wave passes through the detector. As laser/optical/computing technology matured, starting in the 1990s, it was realized that using these devices and using matched filtering data analysis techniques on a large scale would be possible. LIGO was constructed in the late 1990s, and had its initial observing campaigns in the 2000s. Advanced LIGO was a major upgrade of all internal laser, optical, and suspension elements of the detector. This improved the sensitivity of LIGO to the point where the first gravitational waves could be detected from merging pairs of black holes and now neutron stars. The Advanced Virgo detector in Italy, a joint effort of France and Italy along with other European nations, has a similar design to Advanced LIGO, but is slightly smaller than the LIGO detectors. It will have comparable sensitivity to Advanced LIGO in a few years, but even now the detector was able to make the important discovery possible since it allowed us to tell electromagnetic astronomers where to look for the source. We look forward to more sources like these and other new sources of gravitational waves. [Evan Goetz, Scientist, LIGO instrumentalist / astrophysics]

MCB: Does LIGO have plans for how to better detect gravitational waves in the future? Would more detection sites or different currently-available equipment help?

More detectors would help with both identifying the origin of a gravitational wave signal and network uptime.

By timing the arrival of the signal at the two detectors LIGO can roughly point to an area of the sky where the signal originated, but with three or more detectors a better estimate of the location of the source can be made. When the Virgo gravitational wave detector joined the observing run it became possible to produce significantly more accurate ‘maps’ of gravitational wave signals.

Also with more detectors we can observe for longer periods in case one detector goes offline.

Currently the KAGRA detector is under construction in Japan and there are plans underway to construct a third LIGO detector in India.

Both LIGO observatories are currently begin upgraded to increase the sensitivity of the detectors for the next observing run. This includes increasing the laser power, making upgrades to the suspension systems and the installation of a quantum noise mitigation scheme called squeezed light injection.

It is expected that this series of upgrades will improve the sensitivity of the LIGO detectors by a factor of 2, meaning we can search a volume of space 8 times greater, so expect even more detections in the next observing run.

[AS. PhD Student LIGO Instrumentation]

MCB: What’s next for LIGO?

The Laser Interferometer Gravitational-wave Observatory (and other gravitational wave instruments around the world, currently including Virgo, as well as KAGRA and LIGO-India in the coming years, and for exceptionally close sources, GEO600) will serve as just that: an observatory. LIGO will continue to get more sensitive, and as it does, we will observe gravitational wave sources and expand our catalog of known black holes and neutron stars. We’ll also be on the lookout for new types of signals from sources like pulsars, supernovae, and a stochastic background of gravitational waves, as well as gravitational wave sources we may not expect! Future observations will help us learn even more about the Universe and the laws that govern it. [JM, PhD, LIGO data analysis/instrumentation]

MCB: What’s one thing in particular that you wish people knew about gravitational waves?

Better late than never, here’s two times “one thing” from different people:

Observing gravitational waves for the first time is like finally being able to hear the symphony of the Universe and observe the dynamics of the most energetic events. [Evan Goetz, Scientist, LIGO instrumentalist / astrophysics]

Especially since the last publication: we can now predict WHEN and WHERE EM events will happen. We kind of knew a minute in advance when a Kilonova was to happen. and where. That’s absolutely amazing in my opinion. If we get the data flow faster from detection algorithm telescope pointing, we could catch it in time. [Simon Barke, Postdoctoral Researcher, University of Florida]

Special thanks to the following scientists for answering my questions during the Reddit AMA:

Avneet Singh, Albert-Einstein-Institut (c)

Jennifer Wright, University of Glasgow

Noah Sennett, Albert-Einstein-Institut

Hunter Gabbard, University of Glasgow

Simon Barke, University of Florida

Evan Goetz, University of Michigan (LHO)

Rachel Hamburg, University of Alabama Huntsville (FermiGBM)

Jess McIver, Caltech

Andrew Spencer, University of Glasgow

Shivaraj Kandhasamy, University of Minnesota (LLO)

Editor’s Note: This post has been updated to include late, but very welcome, submissions by the LIGO team. More science is always appreciated!

Mike, I don’t know where you come up with stuff like this, but it’s pretty cool. Amazing to see how God created such an intricate universe.
If I’m reading this correctly, one will never get to feel a gravitational wave since one would be torn apart before feeling it. Ouch!
Do you know how often a wave event passes through the earth? Thanks for the post.

Thanks! And good question! Apparently these waves pass through the Earth fairly often, but scientists have only been able to detect them fairly recently. This was the first major event that they were able to observe.